1. Field of the Invention
The inventions disclosed and taught herein relate generally to mechanical counterbalances, and more specifically are related to pneumatic counterbalances suitable for use in machinery, such as linear rod pumping units.
2. Description of the Related Art
Beam pumping units and their upstream drive components are exposed to a wide range of loading conditions. These vary by well application, the type and proportions of the pumping unit's linkage mechanism, and counterbalance matching. The primary function of the pumping unit is to convert rotating motion from the prime mover (engine or electric motor) into reciprocating motion above the wellhead. This motion is in turn used to drive a reciprocating down-hole pump via connection through a sucker rod string. An example of a conventional pumping unit arrangement is illustrated generally in FIG. 1, and will be discussed in more detail herein.
The “4-bar linkage” comprising the articulating beam, pitman, cranks, and connecting bearings processes the well's polished rod load into one component of the gear box torque (well torque). The other component, counterbalance torque, is adjusted on the pumping unit to yield the lowest net torque on the gearbox. Counterbalance torque can be adjusted in magnitude but typically not in phase (timing) with respect to the well load torque. In crank balanced machines, counterbalance torque will appear sinusoidal as it is effectively a mass being acted on by gravity while rotating about a fixed horizontal axis. The basic computation for pumping unit crankshaft torque is:Tnet=Twell−Tebal 
Counterbalance may be provided in a number of forms ranging from beam-mounted counterweights, to crank-mounted counterweights, to compressed gas springs mounted between the walking beam and base structure to name only a few. The primary goal in incorporating counterbalance is to offset a portion of the well load approximately equal to the average of the peak and minimum polished rod loads encountered in the pumping cycle. This technique typically minimizes the torque and forces at work on upstream driveline components reducing their load capacity requirements and maximizing energy efficiency.
Well loads at the polished rod are processed by the 4-bar linkage into crankshaft torque at varying ratios depending on the relative angles of the 4-bar linkage members (i.e. stroke position). Simultaneously, the counterbalance torque produced by one of the various methods above interacts with the well load torque negating a large percentage of it. The resulting net torque exposed to the crank shaft is usually only a small fraction of the original well load torque. Note in the diagram at right that well torque (the component of net torque resulting from the polished rod load) is highly variable, both in magnitude and phase angle (timing). In contrast, the counterbalance torque is smooth and sinusoidal. Its phase angle is established as an attribute of the pumping unit design selected for broadest applicability—and is generally not adjustable. Magnitude and phase angle mismatches between well and counterbalance torque curves are the source of “lumpiness” in the net torque transmitted through the gear reducer and up-stream driveline elements. These elements must be selected with sufficient capacity to survive the peak load conditions encountered during the pumping cycle. Given that the actual pumping work performed during the cycle is equivalent to:WORK=∫Tnetdθit is evident that the “lumpiness” in the net torque curve results in inefficient utilization of the capacity of these driveline elements. Indeed, the net torque curve in the above example dips into negative (regenerative) values in multiple locations during the cycle further reducing the net work performed.
The chief source of variability in the well torque curve is the elastic response of the sucker rod string to dynamic loads transmitted through it from the down-hole pump and the surface pumping unit. The rod string, sometimes miles in length, behaves over long distances similarly to a spring. It elongates when exposed to tensile stress and when the stress is variable, the response is often oscillatory in nature. The system is damped somewhat due to its submergence in a viscous fluid (water and oil) but the motion profile of the driving pumping unit combined with the step function loading of the pump generally leaves little time for the oscillations to decay before the next perturbation is encountered.
The diagram shown in FIG. 3 illustrates generally some of the interactions at work in a typical rod pumping chain. The surface pumping unit imparts continually varying motion on the polished rod. The connecting sucker rod string, modeled as a series of springs, masses, and dampers, responds to accelerations at the speed of sound sending variable stress waves down its length to alter its own motion. It also stretches as it builds the force necessary to move the down-hole pump and fluid. The pump, breaking away from the effects of friction and fluid inertia tends to rebound under the elastic force from the sucker rods initiating an additional oscillatory response within the string. Traveling stress waves from multiple sources interfere with each other along the rod string (some constructively, others destructively) as they traverse its length and reflect load variations back to the surface pumping unit where they can be measured and graphed as part of the surface dynamometer card. The resulting surface dynamometer card, such as the general example in FIG. 4, shows superimposed indications of large scale rod stretching, damped oscillations, friction, as well as inertial effects all in varying amounts depending on the well application and pumping unit geometry.
Problem Addressed: Fixed proportion 4-bar linkage geometries found in typical beam pumping units exhibit application preferences for a relatively narrow band of operating conditions (i.e. conventional units for upward sloping dynamometer cards, Mark II for downward sloping cards, Reverse Mark for level cards, etc). These preferences are fundamental to a particular linkage geometry and are very difficult to change. This is not to say that a Mark II pumping unit (Lufkin Industries, Inc.) cannot operate with an upward sloping card, merely that an optimal efficiency preference exists and that performance consequences are created when they are not obeyed. The diagrams in FIGS. 5 and 6 provide some illustration of this point. Permissible load diagrams (PLD) for similarly sized and counterbalanced Conventional and Mark II (Lufkin Industries, Lufkin, Tex.) pumping units are shown along with a surface dynamometer card for comparison in FIG. 5. Permissible load diagrams display the polished rod load that would be required to create crankshaft torque equivalent to the gear reducer torque rating for a given pumping unit design and counterbalance setting. It can be observed from the shape of the permissible load diagrams in FIG. 5 that the conventional pumping unit exhibits a preference for dynamometer cards with an upward sloping trend (moving from left to right). Conversely, as shown in both FIG. 5 and FIG. 6, the Mark II unit shows a preference for cards that slope downward. The dynamometer card in this instance also shows a slight upward trend causing it to conform somewhat better to the PLD of the conventional unit. Note that both pumping units would be operating at near their up-stream driveline capacities, given the relative proximity of the peak and minimum polished rod load to their respective PLDs.
However, the area of the Mark II unit PLD is substantially larger than that of the Conventional unit indicating that it is capable of performing more work during its pumping cycle. The extra available work capacity of the Mark II pumping unit would be underutilized in this particular application.
An unfortunate reality is that rod pumping dynamometer cards are almost never the vaguely hourglass shape that would maximize the work potential of most beam pumping units, at least not under the near constant rotating velocity conditions under which they have been designed to operate.
Automation technologies for rod pumping applications have existed for a number of years. Operating wells can be monitored by an assortment of methods to collect load and motion information at the surface, then, by computer simulation, diagnose such things as overload conditions or the onset of down-hole issues ranging from pump-off (incomplete pump fillage) to rod buckling to worn or damaged equipment. The predictive simulations performed by many of these rod pump control (RPC) systems are able to accurately model the elastic-dynamic behavior of the rod pumping chain (pump, rods, and pumping unit) with relatively minimal program data entry.
More recently, variable speed drives (VSD) have been integrated with rod pumping unit applications and in conjunction with RPC technology, have markedly improved the longevity and efficiency of many rod pumping systems. Today, it is relatively common to see operating pumping units being monitored by RPCs which can sense system anomalies and send corrective action commands to a VSD to, for example, adjust pumping speed down in response to detected pump-off conditions or possibly to shut down in response to excessive loading. If used in conjunction with supervisory control and data acquisition (SCADA) technology, a well and rod pumping system can be monitored and controlled remotely making it possible to identify and respond to potential equipment maintenance issues or change production goals from a control center miles or perhaps continents away.
The relatively poor pumping unit capacity utilization portrayed in the case above might be at least partially remedied through active speed control. Pumping unit dynamometer cards tend to be fairly repetitive from cycle to cycle and speeding up or slowing down at strategic points within the cycle could influence the shape of the dynamometer card to either truncate load spikes, improve driveline capacity utilization, increase production, or improve system efficiency. Active control of the pumping unit's force/motion profile could also yield significant benefit in terms of rod, tubing, and down-hole pump life. In certain instances, such as with the use of fiberglass sucker rods, RPC and VSD technology could be used jointly with goal seeking algorithms, actively controlling the motion profile to produce large down-hole pump displacements while simultaneously protecting the rod string from the onset of buckling as an example.
Unfortunately, the flywheel effect produced by massive rotating components within the pumping unit resists rapid changes in speed. Cranks, counterweights, gears, sheaves, brake drums and other rotating components in the system contribute to the overall flywheel effect and require significant torque exertion to alter their rotating speed. This presents a substantial impediment to active control scenarios such as those mentioned above. Attempts to substantially alter speed within the pumping cycle with a VSD to date have generally consumed disproportionately more power which negatively affects operating cost. Pumping unit designs with substantially reduced mass moments of inertia appear to be a prerequisite to fully implementing active speed control in rod pumping.
Mass based counterbalance systems present problems in continually maintaining optimum counterbalance as well conditions change. Fluid level in the casing annulus of the well tends to decline with production over time. As fluid level drops, the rod pumping system must lift the fluid from greater depth increasing the amount of counterbalance needed. Conversely, if the well is shut in for an extended period of time, fluid level will typically rise, reducing the needed counterbalance proportionally. Failure to maintain proper counterbalance can lead at best to inefficient power usage and at worst to upstream equipment failures due to overload. Generally, counterbalance adjustments on existing beam unit designs are performed manually by repositioning, adding or removing counterweights in an equipment and labor intensive process requiring unit shut-down and restraint, entry into a hazardous area, use of expensive cranes and equipment, and temporary loss of production to the operator.
Changing stroke length is also a manual process involving the same steps as those above (unit must be re-balanced following a stroke change) with the notable additions that the pumping unit must be decoupled from the well load, crank pins must be driven out and shifted to another hole in the crank arm, crank arms must be re-positioned by crane during re-stroking and the down-hole pump must be re-spaced, also by crane, prior to restoring to service.
Down-hole pump valve testing (valve checks) is generally accomplished by halting the pumping unit's motion on the up-stroke or down-stroke and measuring the rate at which polished rod load declines or rises as a means of assessing leakage rates in the pump's valving. The method of testing typically requires the use of a portable dynamometer and insertion of a calibrated load cell between the carrier bar and rod clamp.
Large and heavy moving parts at near ground level requires relatively extensive safety guarding to prevent inadvertent contact with personnel while the pumping unit is in motion.
The inventions disclosed and taught herein are directed to adaptable surface pumping units that include and combine automation technology with a low inertia pumping unit mechanism capable of responding to active control commands from a well management automation system, thereby allowing the surface pumping unit to change in reaction to changing well conditions, the pumping unit being capable of self-optimization, self-protection, and of safeguarding expensive down-hole equipment, while at the same time presenting a small environmental footprint designed such that typical safety hazards are eliminated or reduced, minimizing the need for warning signage. Such pumping unit systems may further automatically altering and maintaining counterbalance force by controlling the addition or elimination of fluid (e.g., air) mass from a containment vessel associated with the pumping unit.